It has become increasingly desirable to improve cooling systems in aerospace applications. Typically, cooling systems provide air conditioning, refrigeration and freezer services, and the like for commercial and other aerospace systems. In general, various known options are available for providing cooling, but such options have drawbacks that limit the design options for aerospace applications.
One known option includes a vapor compression cycle. Vapor compression cycles pass a refrigerant through two-phase operation and can operate efficiently and take advantage of the thermal carrying capacity of a liquid, as opposed to a gas, as well as take advantage of the heat of vaporization of the liquid refrigerant. Thus, through portions of the vapor compression cycle, the cooling system can be much more compact when compared to a gas or air-based system because the fluid being carried is in liquid form. However, vapor compression cycles typically are limited to lower ambient temperature operation and may not provide useful solutions for high ambient temperature operation.
Another known option is a single-phase gas-based system using a gas such as air as the refrigerant. However although air can serve usefully as a refrigerant medium, air is not an efficient thermal fluid, as its heat capacitance is limited to a function of its mass flow rate and heat capacity. Thus, gas-based systems are typically less efficient than vapor compression systems and are typically, for that reason alone, larger than vapor compression systems. Additionally, air systems typically include significant duct passages in order to carry the amount of air that is desired to achieve the amount of cooling typically used for aerospace purposes.
To accommodate the wide range of possible ambient operating conditions of the aircraft, cooling systems for aerospace applications typically use a gas-based system. That is, although it is desirable to reduce mass and bulk in aircraft or aerospace applications, typical cooling systems nevertheless include a more bulky and less efficient gas-based system in order to cover the range of conditions that can be experienced.
Typically, aircraft operate in a range of operating conditions, during which their cooling systems may be required to operate as well. In one example, the aircraft may reside on a tarmac, such as when taxi-ing for departure, loading passengers or cargo, or awaiting for weather conditions to approve. In another example, the aircraft may be operated at high elevation. In yet another example, the aircraft may be operated in transition and during climbing to elevation. Such operation can present challenges to operation of the refrigeration system, as the condenser and the overall cooling system can be exposed to a wide variety of temperatures and conditions during this range of potential operating conditions.
Other known systems include carbon dioxide (CO2) as a refrigerant which, when operated in trans-critical mode (i.e., spanning operation between super-critical to sub-critical), offer an opportunity to significantly reduce the overall size of the system due to significantly improved system efficiency. The performance of trans-critical systems is very sensitive to refrigerant charge circulating in the main circuit. COP maximum and the cooling capacity depend on ambient and an evaporating temperature. When ambient temperature or the load is changed, the amount of circulating refrigerant should change as well. If the amount of the circulating refrigerant remains the same the operating envelope for ambient and evaporating temperatures may be significantly restricted.
Shortage of refrigerant charge as a result of leakage may cause malfunction of the system, which ranges from underperformance to serious failures. Therefore, diagnosis of the refrigerant inventory in the system and timely the refrigerant charge service is extremely important to maintain robust system performance.
As such, there is a need to improve cooling systems that can operate over a wide range of operating conditions and temperatures.